Elusive tetraquark spotted in a data forest

The keepers at Particle Zoo should ready a new enclosure. Particle hunters are claiming a sighting of a beast of legend – the tetraquark. A jumbo particle made up of four quarks, it is a hitherto undiscovered form of matter.

Tetraquarks were first posited to exist over 30 years ago, as solutions to the equations of quantum chromodynamics. QCD is a theory developed to describe how quarks combine to make two-quark mesons, such as pions and kaons, and three-quark baryons like protons or neutrons.

Although QCD allows for particles made of higher numbers of quarks, such exotic variants have remained in the wild because the physicists hunting them lacked instruments with the power and sensitivity to detect them.

Popping up

In 2003, results from the BELLE collaboration at the KEK particle accelerator in Tsukuba, Japan, hinted at the existence of a pentaquark.

Subsequent analyses of data from other experiments seemed to confirm this, but other groups were unable to recreate Belle's results. As the pentaquark evidence dried up, a number of possible tetraquark sightings have been made by groups at KEK in Japan, the SLAC National Accelerator Laboratory in Stanford, California, and D-Zero at the Fermilab accelerator in Illinois.

Now Ahmed Ali of the German Electron Synchrotron (DESY) in Hamburg, Germany, and his colleagues have found another tetraquark candidate while looking to explain an anomaly reported in 2008. BELLE researchers had collided beams of electrons and positrons in an attempt to create an energetic form of "bottomonium" – a meson made of a bottom quark and its antiparticle. Their product of their collisions, however, decayed much faster than predicted.

Twice as massive

Ali and colleagues found that if a tetraquark made of one bottom quark, one up quark, and each of their anti-particles was created instead of bottomonium, the faster decay could be explained. "This is the sought after tetraquark," says Ali.

The proposed particle is over twice as massive as previous specimens.

According to Ken Hicks of Ohio University in Athens, the team's result supports a growing body of evidence for the tetraquark's existence. "Everyone is dead sure that this particular state exists," he says. However, he adds that there is still debate over whether the team have seen a pure particle or a "meson molecule" – a pair of two-quark mesons orbiting each other.

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Results from the BELLE collaboration at the KEK particle accelerator in Japan hint at a tetraquark (Image: KEK)